The invention relates to controlling the ignition timing of engines used for automobiles, and more specifically to controlling the ignition timing of engines in automobiles provided with lean NOx catalyst.
As is well known, an air/fuel (A/F) ratio in a lean-burn engine having lean NOx catalyst (LNC) is periodically changed to rich side to reduce the NOx occluded by LNC during the lean-burn operation. Another technique is also known for performing ignition timing retard (lagging) while the air/fuel ratio is being kept rich in order to absorb the engine torque that may occur due to rich operation, suppressing the deterioration of drivability (that is, shock) due to possible torque fluctuation.
For example, The Japanese Laid-Open Patent Application No. 7-103015 discloses a method for controlling the amount of intake air during rich reduction process (shift-to-rich process) as well as lagging the ignition timing, to improve the drivability during the rich reduction process.
In those conventional techniques, however, a rich reduction period required for reducing the lean NOx catalyst saturated with NOx may become too long when, for example, the automobile runs at low-speed. If the ignition timing is keep retarded throughout such long rich reduction period, the combustion efficiency may be decreased and consequently the fuel efficiency may become worse.
In order to resolve the aforementioned problem, according to one aspect of the invention, an electronic control unit is provided, comprising: air/fuel ratio control means for changing the air/fuel ratio to a rich side when the amount of the NOx occluded by the lean NOx catalyst exceeds a predetermined value and changing said air/fuel ratio back to a lean side when a first predetermined period has elapsed; lagging means for lagging the ignition timing in synchronization with the air/fuel ratio change to the rich side by said air/fuel ratio control means; and decrementing means for decrementing by a predetermined amount the lagged amount of the ignition timing by said lagging means when a second predetermined period shorter than said first predetermined period has elapsed after the air/fuel ratio change to the rich side.
The ignition timing is lagged in synchronization with the air/fuel ratio changing to the rich side and the lagged amount of the ignition timing is decremented after the predetermined period has elapsed. Therefore, it is possible to prevent the decrease of the fuel efficiency without deterioration of the drivability due to the rich operation for reducing the NOx catalyst. In one preferred embodiment of the invention, said first predetermined period is a period that elapses before an accumulated value SRSNT1 of intake air amount correlation value reaches a threshold value, and said second predetermined period corresponds to a value TIGRSR, which will be discussed later.
According to another aspect of the invention, said decrementing means of the electronic control unit gradually decrements said lagged timing amount up to a hold value. Because the lagged timing amount decrements gradually up to the hold value, it is possible to prevent the decrease of the fuel efficiency without the deterioration of the drivability.